1. Field of the Invention
The present invention relates to an apparatus and method for measuring an aperture size of a near-field optical probe, and more particularly, to an apparatus and method for measuring an accurate aperture diameter of a near-field optical probe using a filter. The present application is based on Korean Patent Application No. 2001-57575, which is incorporated herein by reference.
2. Description of the Related Art
Near-field optical probes are generally used in apparatuses for near-field optical microscopy such as high resolution apparatuses for measuring material surfaces or superhigh density recording media.
In an apparatus for measuring resolution of a material surface, resolution R is defined as the distance between two points that can be resolved and is given by equation 1, wherein the resolution R is proportional to a wavelength xcex and inversely proportional to an aperture diameter of a lens or an iris.                     R        =                              1.22            ⁢            λ                    D                                    (        1        )            
In conventional far-field optical microscopy, resolution has to be small as the wavelength of light becomes shorter. However, there is a limit beyond which the resolution cannot be made small due to the diffraction of light. This diffraction limit is not encountered in near-field scanning optical microscopy (hereinafter referred to as xe2x80x9cNSOMxe2x80x9d). Thus, it is possible to manufacture a high resolution apparatus for measuring a material surface.
In NSOM, it is necessary to accurately know an aperture size of a near-field optical probe in order to measure high resolution of a sample which is positioned at the near-field optical probe of a size smaller than a wavelength, i.e., a sub-wavelength size, using a near-field optical microscope.
An aperture diameter of the optical probe in the NSOM has a subwavelength-size, i.e., a diameter d of about 50-300 nm in near-field optical microscopy using a visible ray with a wavelength xcex of 400-1000 nm. This subwavength resolution can be achieved when the sample is positioned at the near-field optical probe.
Conventional NSOM uses a scanning electron microscope (SEM) or a measurement only apparatus (disclosed in U.S. Pat. No. 5,663,798) in order to obtain the aperture diameter of the optical probe.
SEM reduces an electron beam generated from an electron gun to a diameter within a range of several to hundreds xc3x85, using several electron lenses, radiates it onto a sample, detects secondary electrons emitted from the sample or electrons having passed through the sample, modulates brightness in time series on an oscilloscope, and measures the surface of the sample.
SEM can accurately measure an aperture of an optical probe, but it is expensive and takes a long time.
To measure the aperture of an optical probe using SEM, the tip of the optical probe, which is an electrical insulator, has to be coated with a conductive material. When the aperture of the coated optical probe is measured and then the coated optical probe is used in NSOM, the coating of the tip of the optical probe degrades the performance of NSOM. Thus, since it is difficult to reuse the optical probe measured with SEM, a new optical probe should be used in NSOM. However, the new optical probe may have a different diameter than the optical probe measured with SEM.
FIG. 1 is a schematic view of an apparatus for measuring an aperture size of an optical probe disclosed in U.S. Pat. No. 5,663,798. Referring to FIG. 1, the apparatus includes a light source 11 for radiating light, a polarizer 13 for polarizing the light, a focusing lens 15 for focusing the light onto an optical probe 10, a linear analyzer 17 for collecting light transmitted through the optical probe 10 through an optical detector 19, and the optical detector 19 for converting the light into an electrical signal and detecting the electrical signal.
In the apparatus, to deduce an aperture diameter, the light transmitted through the aperture of the optical probe 10 at a predetermined angle is received, a signal corresponding to light intensity is detected from the optical detector 19, and the signal is transmitted to a data acquisition unit (DAU) 23 or a computer (PC) 25.
As shown in FIG. 1, a motor 21 in which the linear analyzer 17 and the optical detector 19 are built rotates from xe2x88x92165xc2x0 to +165xc2x0 to measure the angular light intensity distribution emitted from the aperture in far-field. As a result, the aperture diameter of the optical probe can be measured.
FIG. 2 is a graph showing an angular distribution light intensity measured by the conventional aperture measuring apparatus. Referring to FIG. 2, if the wavelength of the light emitted from the light source 11 is 633 nm and a polarization angle is 90xc2x0, the far-field angular intensity distribution of light transmitted through the optical probe having an aperture diameter of 60 nm, 380 nm, or 3.2 xcexcm (which is pre-measured with SEM) is gaussian with a maximum light intensity value at 0xc2x0.
As can be seen from FIG. 2, as the aperture diameter decreases, a full width at half maximum (FWHM) becomes increasingly wider. Here, the FWHM is the difference between two angles corresponding to half of the maximum light intensity value.
Referring to FIG. 2, if an aperture diameter d (=2a) of the optical probe is 60 nm, the FWHM is the difference between two angles +60xc2x0 and xe2x88x9260xc2x0 corresponding to the light intensity of 0.5, i.e., 120xc2x0. If the aperture diameter d (=2a) of the optical probe is 380 nm, the FWHM is the difference between +30xc2x0 and xe2x88x9230xc2x0, i.e., 60xc2x0.
FIG. 3 is a graph of FWHM according to aperture diameters of the optical probe with reference to FIG. 2. In FIG. 3, the line (a) is predicted based on Kirchoff""s theory, the line (b) is given by the small aperture limit theory of Bethe, and the line (c) is that given by the conventional measuring apparatus.
The aperture diameter of an optical probe can be obtained by obtaining the FWHM from the intensity distribution of the light emitted from the optical probe using FIG. 3 and then finding the corresponding diameter from FIG. 2.
The apparatus shown in FIG. 2 requires an additional unit which rotates around the optical probe to measure the light intensity transmitted through the optical probe. And, if the rotation of the apparatus is not precise, it is difficult to measure an accurate aperture diameter. Also, since it is difficult to position the tip of the optical probe accurately at the center of rotation, errors easily occur when measuring the aperture diameter.
Moreover, the light intensity should be measured and graphed at a plurality of angles with the rotation of the motor. Thus, it takes a long time to measure aperture diameters and it is difficult to measure aperture diameters smaller than xcex/6 due to measurement limit actions of the apparatus.
To solve the above-described problems, it is an object of the present invention to provide an apparatus for accurately measuring the aperture of an optical probe without damaging the optical probe which can easily be manufactured and configured, and a method thereof.
Accordingly, to achieve the above object, there is provided an apparatus for measuring an aperture of a near-field optical probe. The apparatus includes a light source, an optical detector, and a filter. The light source radiates light to the near-field optical probe. The optical detector is positioned before the near-field optical probe and receives the light transmitted through the near-field optical probe to detect light intensity. The filter is prepared between the light source and the optical detector and transmits only light of wavelengths in a specific mode from the light transmitted through the near-field optical probe.
Here, if a free space or a medium exists between the light source and the filter, the specific mode is a Bessel Gauss mode.
The free space, which has a refractive index of 1, is one of media having uniform refractive indexes.
To achieve the above object, there is provided an apparatus for measuring an aperture of a near-field optical probe. The apparatus includes a light source, an optical detector, a filter, and an optical waveguide. The light source radiates light to the near-field optical probe. The optical detector is positioned before the near-field optical probe and receives the light transmitted through the near-field optical probe to detect light intensity. The filter is prepared between the light source and the optical detector and transmits only light of wavelengths in a specific mode from the light transmitted through the near-field optical probe. The optical waveguide is disposed between the near-field optical probe and the filter and transmits the light.
If the optical waveguide is a graded-index waveguide, the specific mode is Hermite-Gauss mode.
The specific mode is a Laguerre-Gauss mode if the optical waveguide is a graded-index fiber.
The specific mode is a step-index waveguide mode or a step-index fiber mode if the optical waveguide is a step-index waveguide or a step-index fiber.
To achieve the above object, there is provided an apparatus for measuring an aperture of a near-field optical probe. The apparatus includes a light source, an optical detector, a filter, and a mask. The light source radiates light to the near-field optical probe. The optical detector is positioned before the near-field optical probe, and receives the light transmitted through the near-field optical probe to detect light intensity. The filter is prepared between the light source and the optical detector and transmits only light of wavelengths in a specific mode from the light transmitted through the near-field optical probe. The mask is disposed between the light source and the filter and has a cavity in the center through which the light passes.
The specific mode is a mask mode.
To achieve the above object, there is provided a method of measuring an aperture of a near-field optical probe using a filter for transmitting light in a specific mode. The method includes steps: (a) radiating light having a predetermined wavelength to an optical probe; (b) transmitting wavelengths in a specific mode from the light emitted from the near-field optical probe, using a filter; (c) detecting a first light intensity value from a first far-field light intensity distribution of light of wavelength having a mode number of zero from the light transmitted through the filter; (d) detecting a second light intensity value from a second far-field light intensity distribution of light of wavelength having a mode number not zero from the light transmitted through the filter; and (e) substituting a ratio of the first and second light intensity values in a predetermined equation with respect to an aperture diameter of the near-field optical probe to obtain the aperture diameter of the near-field optical probe.
Step (b) further includes transmitting the light transmitted through the near-field optical probe to the filter through a predetermined medium.
Here, if the predetermined medium has a uniform refractive index, the specific mode is a Bessel Gauss mode.
If the predetermined medium is a graded-index waveguide, the specific mode is a Hermite-Gauss mode.
If the predetermined medium is a graded-index fiber, the specific mode is a Laguerre-Gauss mode.
If the predetermined medium is a step-index waveguide or a step-index fiber, the specific mode is a step-index waveguide mode or a step-index fiber mode.
Step (b) may further include transmitting the light transmitted through the near-field optical probe to the filter through a mask having a cavity in the center. Here, the specific mode is a mask mode.
Step (d) may include obtaining a second light intensity value from a second far-field light intensity distribution of a wavelength having a mode number of 2 from the light transmitted through the filter.
Step (e) includes steps: (e-1) obtaining a mode solution corresponding to a specific mode according to a specific medium; (e-2) calculating a coupling constant corresponding to the mode solution and obtaining relationship equation with respect to the optical probe aperture diameter of the coupling constant; and (e-3) substituting a ratio of the first light intensity value measured in step (c) and the second light intensity value measured in step (d) for the relationship equation to deduce the aperture diameter of the near-field optical probe.
In step (e-1), the specific mode is a Hermite-Gauss mode if the specific medium is a graded-index waveguide.
In step (e-2), an equation with respect to the aperture diameter of the near-field optical probe of the coupling constant corresponding to the mode solution is obtained.
In step (e-3), the aperture diameter of the near-field optical probe can be deduced from a predetermined equation if the second light intensity value measured in step (d) corresponds to a wavelength having a mode number of 2.
When optical information is stored using an optical probe with an aperture diameter smaller than a wavelength of light or a sample is observed with a microscope having the optical probe, it is necessary to accurately know the aperture diameter of the optical probe to reproduce written information or an accurate image on the surface of the sample.
In the present invention, the aperture diameter can easily be measured in comparison to a conventional method of measuring an aperture diameter of a near-field optical probe, i.e., a method using a SEM or a separate measuring apparatus. Also, since the aperture diameter can be measured without damaging the near-field optical probe, a corresponding optical probe can be selected. Further, the configuration of the apparatus according to the present invention is simple compared to a conventional apparatus and the measuring cost can be reduced. Moreover, it is possible to measure aperture diameter sizes of xcex/6 or less which are difficult to measure by a conventional method.